In general, electrostatic printing processes typically involve creating an image on a photoconductive surface, applying an ink having charged particles to the photoconductive surface, such that they selectively bind to the image, and then transferring the charged particles in the form of the image to a print substrate.
Electrostatic printing systems will now be described in a little more detail. The photoconductive surface is typically on a cylinder and is often termed a photo imaging plate (PIP). The photoconductive surface is selectively charged with a latent electrostatic image having image and background areas with different potentials. For example, an electrostatic ink composition comprising charged toner particles in a carrier liquid can be brought into contact with the selectively charged photoconductive surface. The charged toner particles adhere to the image areas of the latent image while the background areas remain clean. The image is then transferred to a print substrate (e.g. paper) directly or, more commonly, by being first transferred to an intermediate transfer member, which can be a soft swelling blanket, and then to the print substrate. Variations of this method utilize different ways for forming the electrostatic latent image on a photoreceptor or on a dielectric material.
Typically, an electrostatic ink composition comprises a thermoplastic resin as the basis for the toner particles, and a non-polar liquid as a carrier liquid in which the toner particles are dispersed. Generally, the toner particles contain a colorant such as a pigment. A charge director, also called charge control agent or imaging agent, is also typically added to the dispersion to induce charge on the particles.
In some systems, the electrostatic ink composition is applied to the PIP by pumping the ink between a stationary electrode and the PIP. Such a system is illustrated in WO 2005/054959, which is incorporated herein by reference in its entirety. Other electrostatic printing systems include a binary ink development (BID) unit. In such a system, the ink is applied to the photoconductive surface by a developer roller. Often, a different developer roller is used for each different color ink (e.g. cyan, magenta, yellow and black). Ink is applied to the developer roller by passing an electrostatic ink composition between a stationary charged electrode and the developer roller. Ideally, the charged toner particles should form a uniform layer on the development roller. The developer roller rotates, such that the charged particles contact the PIP electrically. Such a system is illustrated in U.S. Pat. No. 5,436,706, U.S. Pat. No. 5,610,694 and U.S. Pat. No. 5,737,666, all of which are incorporated herein by reference in their entirety.
The present inventors found that some electrostatic inks suffered from the problem of optical density reduction (if developing voltage is kept constant) or a rise in developing voltages (if developing voltage is corrected to maintain the same optical density). An increase in the drop in optical density is considered to correlate with an increase in the charge on the resin particles. The problem was observed on low coverage printing and became severe after only a few hundred impressions. The problem seemed particularly acute with inks containing organic pigments with conjugated aromatic rings, such as phthalocyanines, which are often included in cyan inks.